A. Field of the Invention
The invention relates to highly erosive and abrasive wear resistant composite coating. More particularly, the invention relates to an improved highly erosive and abrasive wear resistant coating comprising a composite coating system of an intermediate layer of substantially pure tungsten and an outer two phase layer of a mixture of tungsten and tungsten carbide.
B. Background Art
High hardness materials are widely used as coatings on various type of mechanical components and cutting tools. Such coatings impart erosion and abrasion wear resistance and thus increase the erosive and abrasive wear life of objects that have been coated. The high hardness materials can also be used to produce free standing objects which are erosive and abrasive wear resistant.
Chemical vapor deposition processes can be used to produce highly erosive and abrasive wear resistant hard coatings and free standing objects. In a typical chemical vapor deposition (CVD) process, the substrate to be coated is heated in a suitable chamber and then a gaseous reactant mixture is introduced into the chamber. The gaseous reactant mixture reacts at the surface of the substrate to form a coherent and adherent layer of the desired coating. By varying the gaseous reactant mixture and the CVD process parameters, various types of deposited coatings can be produced.
In co-pending U.S. patent application Ser. No. 92,809, filed Sept. 3, 1987, extremely hard, fine grained, non-columnar, substantially lamellar tungsten/carbon alloys are described which are produced by chemical vapor deposition. The described alloys consist primarily of a mixture of a substantially pure tungsten phase and at least one carbide phase wherein the carbide phase consists of W.sub.2 C or W.sub.3 C or a mixture of W.sub.2 C and W.sub.3 C. The disclosed tungsten/carbon alloys are free of columnar grains and consist essentially of extremely fine, equiaxial crystals.
It has been found that the tungsten/carbon alloys such as those described in the aforementioned U.S. patent application, when deposited upon certain types of substrates, exhibit a very fine micro-crack system throughout the deposit. On many types of substrates and under many types of erosive and abrasive wear conditions, preferential attack occurs at the cracks, resulting in poor erosion and abrasion wear resistance for such coatings.
The use of an intermediate layer of substantially pure tungsten followed by a tungsten carbide coating is described in the prior art. For example, U.S. Pat. No. 3,389,977 discloses a method of depositing substantially pure tungsten carbide in the form W.sub.2 C wherein the adherence of W.sub.2 C to a steel substrate is improved by first cleaning the surface and then depositing a thin film of tungsten. The thin film of tungsten is deposited at or above 600.degree. C., making the use of the deposition process unsuitable for providing erosive and abrasive wear resistance coating on various carbon steels, stainless steels, nickel and titanium alloys without severely degrading their mechanical properties. Additionally, pure W.sub.2 C deposited according to this patent consists of columnar grains as opposed to non-columnar grains described in the present patent application. Other instances of the use of very thin tungsten intermediate layers, often as a diffusion layer, are reported in other prior art in order to improve adhesion of tungsten carbide on a substrate. However, there is no report in the prior art of the effect of a tungsten interlayer on coating properties of the final coating system nor has the effect of such a tungsten intermediate layer on the reduction or elimination of cracks in the outer coating been reported.